In order to prevent the inadvertent opening of a pressure vessel while it is under pressure, it is common practice to mount a manually operable relief valve on the cover of the vessel. In accordance with normal operating procedure the valve is first opened to connect the chamber in the vessel to the ambient, and then the hold down bolts are loosened to permit the cover to be opened. Because of unfamiliarity with this procedure or because of carelessness, the bolts have been loosened without first opening the manual relief valve, thereby subjecting the person opening the cover as well as others to possible danger.